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Iterators
An iterator permits you to examine
the elements of a data structure one
at a time.
Iterator Methods
Iterator ix = x.iterator();
constructs and initializes an iterator to
examine the elements of x;
constructed iterator is assigned to ix
you must define the method iterator
in the class for x
Iterator Methods
ix.hasNext()
returns true iff x has a next element
ix.next()
throws NoSuchElementException if
there is no next element
returns next element otherwise
Optional Iterator Method
ix.remove()
removes last element returned by
ix.next()
throws UnsupportedMethodException if
method not implemented
throws IllegalStateException if ix.next()
not yet called or did not return an
element
Using An Iterator
Iterator ix = x.iterator();
while (ix.hasNext())
examine(ix.next());
vs
for (int i = 0; i < x.size(); i++)
examine(x.get(i));
Merits Of An Iterator
• it is often possible to implement the
method next so that its complexity
is less than that of get
• many data structures do not have a
get by index method
• iterators provide a uniform way to
sequence through the elements of a
data structure
Java’s Array Linear List Class
java.util.ArrayList
Cadillac version of our
ArrayLinearListWithIterator
Linked Representation
• list elements are stored, in memory,
in an arbitrary order
• explicit information (called a link)
is used to go from one element to
the next
Memory Layout
Layout of L = (a,b,c,d,e) using an array representation.
a b
c
d
e
A linked representation uses an arbitrary layout.
c
a
e
d
b
Linked Representation
c
a
e
d
firstNode
pointer (or link) in e is null
use a variable firstNode to get to the
first element a
b
Normal Way To Draw A Linked List
firstNode
null
a
b
c
link or pointer field of node
data field of node
d
e
Chain
firstNode
null
a
b
c
d
e
•A chain is a linked list in which each node
represents one element.
• There is a link or pointer from one element to
the next.
• The last node has a null pointer.
Node Representation
package dataStructures;
class ChainNode
{
// package visible data members
Object element;
ChainNode next;
// constructors come here
}
next
element
Constructors Of ChainNode
null
ChainNode() {}
ChainNode(Object element)
{this.element = element;}
null
null
element
ChainNode(Object element, ChainNode next)
{this.element = element;
this.next = next;}
next
element
get(0)
firstNode
null
a
b
c
d
e
checkIndex(0);
desiredNode = firstNode; // gets you to first node
return desiredNode.element;
get(1)
firstNode
null
a
b
c
d
e
checkIndex(1);
desiredNode = firstNode.next; // gets you to second node
return desiredNode.element;
get(2)
firstNode
null
a
b
c
d
e
checkIndex(2);
desiredNode = firstNode.next.next; // gets you to third node
return desiredNode.element;
get(5)
firstNode
null
a
b
c
d
e
checkIndex(5);
// throws exception
desiredNode = firstNode.next.next.next.next.next;
// desiredNode = null
return desiredNode.element; // null.element
NullPointerException
firstNode
null
a
b
c
d
desiredNode =
firstNode.next.next.next.next.next.next;
// gets the computer mad
// you get a NullPointerException
e
Remove An Element
firstNode
null
a
b
c
remove(0)
firstNode = firstNode.next;
d
e
remove(2)
firstNode
a
b
cc
null
d
beforeNode
e
first get to node just before node to be removed
beforeNode = firstNode.next;
remove(2)
firstNode
null
a
b
c
d
e
beforeNode
now change pointer in beforeNode
beforeNode.next = beforeNode.next.next;
add(0,’f’)
firstNode
null
f
a
b
c
d
e
newNode
Step 1: get a node, set its data and link fields
ChainNode newNode =
new ChainNode(new Character(‘f’), firstNode);
add(0,’f’)
firstNode
null
f
a
b
c
newNode
Step 2: update firstNode
firstNode = newNode;
d
e
One-Step add(0,’f’)
firstNode
null
f
a
b
c
d
e
newNode
firstNode = new ChainNode(
new Character(‘f’), firstNode);
add(3,’f’)
firstNode
newNode
f
null
a
c
b
d
e
beforeNode
• first find node whose index is 2
• next create a node and set its data and link fields
ChainNode newNode = new ChainNode(new Character(‘f’),
beforeNode.next);
• finally link beforeNode to newNode
beforeNode.next = newNode;
Two-Step add(3,’f’)
firstNode
newNode
f
null
a
b
c
d
e
beforeNode
beforeNode = firstNode.next.next;
beforeNode.next = new ChainNode(new Character(‘f’),
beforeNode.next);